Platelets and polymorphonuclear neutrophils (PMN) are important effector cells of the hemostatic and inflammatory responses to tissue injury. The dynamics of interaction between platelets and PMN provides a basis for understanding relevant pathophysiologies. Studies in vitro on platelet-PMN interactions have revealed both stimulatory and inhibitory effects on PMN function. We are investigating the intercellular mechanisms of platelet- PMN interactions and the functional consequences of platelet-PMN adhesion. Platelets and PMN were isolated from autologous human blood. Migrated and non-migrated PMN were separated after N-formylmethionyl-leucyl- phenylalanine (FMLP) activation. Platelets were labeled with a fluorescent monoclonal antibody directed against CD41. Platelets (300 million/ml) and PMN (3 million/ ml) were incubated together. Heterotypic cell adhesion was measured in isolated PMN and PMN co-incubated with platelets by flow cytometric analysis of platelet marker fluorescence in PMN gated events. Platelet-PMN adhesion was also visualized by fluorescence microscopy. In studies of isolated PMN, contaminating platelets were bound to 16-34% of un-stimulated PMN, 7-22% of stimulated PMN, 2-4% of migrated PMN, and 17-24% of non-migrated PMN. When platelets were co-incubated with migrated or non-migrated PMN, 15-78% of PMN bound one or two platelets. A preliminary study showed that platelet-PMN co-incubation did not affect the surface expression of adhesion molecules CD11a, CD11b, CD 41 and CD 44 on FMLP-stimulated platelets. Previous studies have shown that their expression is down-regulated differentially on migrated PMN. Our present studies show that un-activated platelets adhere to isolated PMN in vitro. Fewer platelets were adhered to migrated PMN than to non- migrated PMN in isolated PMN preparations. These results indicate that platelets adhering to PMN are removed during PMN migration. The results of this work were published in Clinical Molecular Pathology and the project was completed.